1. Field of the Invention
The present invention relates to an optical pick-up apparatus. More particularly, the present invention relates to a hologram element used in a compatible optical pick-up apparatus for recording data to and/or reproducing data from both digital versatile disc (DVD)-type optical disks and compact disc (CD)-type optical disks.
2. Description of the Related Art
Generally, optical disks are used as storage media for recording and/or reproducing data, such as audio, images and text, by varying the reflection of a laser beam through a plurality of pits formed on the surfaces thereof. The optical disks have a high storage capacity, and are handy to carry, thereby being substituted for conventional storage media such as floppy disks.
Korean Patent Laid-open Publication No. 2003-0097328, which is assigned to the assignee of the instant application and the entire contents of which are hereby incorporated by reference, discloses a conventional optical pick-up apparatus for recording and/or reading optical disks. The optical pick-up apparatus disclosed by the above patent records data to and/or reproduces data from optical disks, such as DVD-RAM/RW/R and CD-RW/R.
The above described optical pick-up apparatus includes a polarized hologram for diffracting light, reflected by the optical disk, into zero-order and positive and negative first-order rays before the beam forms an image onto a photodetector. The polarized hologram diffracts only linear polarized rays in a designated direction. That is, the polarized hologram linearly transmits a P-polarized ray, and diffracts an S-polarized ray. The polarized hologram is divided into five diffraction regions, and serves to split the light forming the image onto the photodetector.
The photodetector includes a plurality of light-receiving portions for generating signals from the beams split by the polarized hologram. The signals detected by the respective light-receiving portions are selectively used to generate a servo control signal and a reproduction signal.
When an optical disk player set including the above optical pick-up apparatus is operated to record data to and/or reproduce data from an optical disk, the optical disk player set reaches a comparatively high temperature due to heat generated from a chip or motor installed therein. Although a cooling fan for exhausting the heat is installed in the set, it is difficult to prevent the temperature from increasing in the optical disk set. Particularly, the high power of a laser beam increases the temperature of the set to 60° C. in a DVD recording mode.
Optical components constituting the optical pick-up apparatus are warped or distorted by the heat deformation when operating at such a high temperature. The warpage or the distortion of the optical components changes the optical path, thereby deviating the light received by the photodetector from a designated position, or distorting an image of the light to deteriorate a signal reproducing capacity. Particularly, it generates an error in detecting a signal for a tracking the servo, thereby increasing the possibility of tracking errors. Hereinafter, with reference to the above Laid-open Patent, the tracking errors will be described in more detail.
Using a push-pull method, a tracking error signal is detected by the difference between a first push-pull signal regarding a detecting signal of second to fifth single signal light-receiving portions and a second push-pull signal regarding a detecting signal of a first quartered light-receiving portion. When the optical pick-up apparatus is operated at a high temperature, images formed on the respective light-receiving portions are deformed. When the images formed on the second to fifth single light-receiving portion are deformed, the amount of the overall light is not changed. However, when the image formed on the first quartered light-receiving portion is deformed, the balance of the optical power in divided regions of the first quartered light-receiving portion is disturbed. Consequently, the second push-pull signal regarding the detecting signal of the first quartered light-receiving portion is changed, thereby generating errors in the tracking error signal.